Method of producing electrically semiconducting coatings



Patented Aug. 10, 1948 METHOD OF PRODUCING ELECTRICALLY SEMICONDUCTING COATINGS Ber-til Stalhane, Stockholm, Sweden, assignor to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, 2. Swedish corporation No Drawing. Application March 14, 1945, Serial No. 582,790. In Sweden March 11, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires March 11, 1964 4 Claims. 1

In many cases, it is useful to provide a. body with an electrically semi-conducting coating. If the body is an insulator, this may be useful for producing high-ohmic resistors, or in such cases, where the surface of the body is in contact with solid, liquid, or gaseous conductors, for obtaining a more uniform electric field distribution along or adjacent to the surface of the insulator than would be possible without such a coating. If the body itself is a semi-conductor, it may be advisable to provide it with a coating, the conductivity of which can be better controlled in detail than the conductivity of the mass of the body. If the body is a good conductor, for instance a metal, its provision with a semi-conducting coating may give a combination presenting the properties of a unilaterally conductive valve or of a voltagedependent resistor.

Electric unilaterally conductive valves of a socalled dry character have hitherto generally consisted of a metal plate with a conductive layer, which has been formed either by a compound, as an oxide, of the metal itself, or of a semi-conduc tive element, as selenium. Recently, proposals have been made to load such valves in their blocking direction with more than their blocking voltage and thereby utilizing their .more or less sharply bent current-voltage characteristic, thus causing them to operate as voltage-dependent resistances, in which case they generally have the advantage over such resistances of a ceramic type that the voltage-dependence becomes marked at lower absolute voltage values. Copper oxide as well as selenium valves have, however, been found to possess certain weaknesses, for instance, they are likely to be destroyed at high temperatures. For instance, copper oxide valves are damaged already at about 50 C. and selenium valves at about 80 C. Especially when the known types of blocking valves are used as voltage-dependent resistors, it has also been felt as an inconvenience, that the current-voltage characteristic is strongly dependent on temperature, ageing and previous operational conditions, for instance if such resistors form part of voltage regulators. In such cases, it may also be desirable, that the individual valves have a lower blocking voltage than the known types, so that the losses when loaded in the blocking direction will be small and the cooling thereby facilitated, while also a finer adjustment of the normal voltage value by putting resistance elements in and out of circuit will be possible.

According to the present invention, an electrically semi-conducting coating On a body consists of a thin layer of at least one metal monoxide and at least one sesquioxide of a trivalent metal. It has been found that such a composition has an electronic conductivity and other electrical properties which are favorable not only as regards the coating in itself, but also in its relation to the basic body. It has also good mechanical properties. The mean value of the conductivity will as a rule be appropriate for use as a semi-conductor, and when applied to a conductive base, a unilaterally conducting valve will be obtained which is well heat-resistant and has a stable voltagecurrent characteristic, in which a sharp bend may occur already at some volts per element. The coating will also obtain a good mechanical strength and shows a good adhesion to different base materials. The expression electronic conductivity is used above to distinguish from electrolytical conductivity (acting by transport of ions) but is intended to cover also such conductivity which depends on a deficit of free electrons, as well as that depending on a surplus of free electrons, the former kind being sometimes referred to as hole conductivity.

Whether the composition of metal oxides used in the coating should be essentially designated as a chemical compound or as a mixture is in some cases uncertain; at any rate the proportion between the component oxides may be different from that corresponding to an exact chemical compound. It has been found advisable thatthe metal forming the sesquioxide has also the property of forming an oxide of so high oxygen content that it has the properties of an acid. Examples of such metals are chromium, molybdenum, tungsten, and manganese. As the monoxidic constituent, the monoxides of copper, nickel, iron, manganese, and cobalt have been found suitable.

In applying the coating to a metallic base, a negative oxide rich in oxygen may be found especially suitable for the reason, that it is likely to form with the basic metal an oxide layer which may be insulating or very little conducting, such a layer being, at least according to certain thecries, of importance for creating a blocking valve action, which for higher voltage values may be transformed into the action of a voltage-dependent resistance. For instance, aluminum or its alloys has been found appropriate as a base metal for this purpose, as its oxide is known as insulating. A similar action may be obtained with a base of chromium or of metal-silicon alloys. At the same time it has been found, that the blocking voltage of such a resistance layer provided on a base metal may be kept so low that the action of a voltage-dependent resistance may be obtained already at some volts.

The manufacturing of a coating composed according to the present invention is preferably carried out in the following manner. If the oxides-1 of the metals in question form together asoluble salt (one of the metals forming then preferably a higher oxide than the sesquioxide acting as the acid constituent of the salt), they are applied to 1 said salt to convert it into an insoluble, electronieally conductive coating.

2. A method of providing a conducting coating on a non-combustible base, which comprises apthe surface in solution. The same may be'the case if the oxides are separately soluble. If only one of them is soluble, the other may be suspended in the solution and applied in, this manner. One or more insoluble oxides may also be suspended in a solution of one or more salts formed by the. a

other ones. The composition is then transformed into an insoluble, electronically conductive form by heat action, during which some oxygen may be transferred either from one or some of the oxides to the other ones, or from one or more oxides to the base metal, or to the air.

As an example of the manufacture, the following may serve. In a solution of one part by weight of cupric dichromate (CuCrzOv) in an equal quantity by weight of water, 0.5-1 part by weight of finely disintegrated, for instance precipitated, cuprous oxide C1120 or manganous oxide 'MnO is suspended. The suspension is applied by a brush or by spraying on a ceramic or metallic base to a thickness of 0.1 to 0.5 mm. The whole is then slowly heated to 250-350 C., whereby the coating is first dried and then transformed into an insoluble composition which probably mainly consists of C110 and C1'2O3, the chromic acid (0103) having given off part of its oxygen to the cuprous oxide to form the cupric oxide (a monoxide) and the chromic oxide (a sesquioxide). In manufacturing voltage-dependent resistances according to this example, aluminum is preferably used as a base.

Conductors provided with coatings according to the present invention may for instance have the form of plates coated on one side or both sides, depending on whether they are intended to be used as unilaterally conducting valves or as symmetrical voltage-dependent resistors. For the latter purpose, the conductors may also be in the shape of grains which are coated separately and afterwards or in conjunction with the heat treatment of the coating are compressed or baked to resistance bodies.

I claim as my invention:

1. A method of providing a conducting coating, which comprises applying on a base of a metal, capable of forming a substantially insulating oxide, containing aluminum as its principal constituent a soluble salt containing, as alkaline conplying on said base, a suspension of an insoluble oxide of a metal of atomic number from 25 to 29, inclusive of limits, in a solution of a salt containing, zas alkaline constituent, an oxide of a metal oi atomic number from 25 to 29, inclusive of limits, and, as acid constituent, an oxide of a metal. selected from the group consisting of chromium, manganese, tungsten and molybdenum,.,drying, and heating said salt to convert it into an insoluble, electronically conductive coating.

3. A method of providing a conducting coating on a non-combustible base, which comprises applying on said base a suspension of cuprous oxide in a solution of copper dichromate, and drying, and heating said dried coating to convert it into an insoluble, electronically conductive coating.

4. A method of providing a conducting coating on a base consisting of grains, which comprises applying on said grains a solution of a salt containing as alkaline constituent an oxide of a metal of atomic number from 25 to 29, inclusive of limits, and as acid constituent an oxide of a metal selected from the group consisting of chromium, manganese, tungsten and molybdenum, drying, and heating and compressing said grains to-convert their coating into an insoluble, electronically conductive coating binding said grains together.

BERTIL STALHANE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 507,082 Raiudnitz Oct, 17, 1893 1,747,776 Jirotka Feb. 18, 1930 1,751,361 Ruben Mar. 18, 1930 2,056,386 Cambron' Oct. 6, 1936 2,298,679 Christensen Oct. 13, 1942 2,301,983 Tanner Nov. 17, 1942 2,329,511 Christensen Sept. 14, 1943 2,370,443 Biefeld Feb. 2'7, 1945 FOREIGN PATENTS Number Country Date 314,769 Great Britain July 2, 1929 

